Present day hydroxyl-terminated polybutadienes in solid propellant formulations are crosslinked with diisocyanates. Earlier developed propellants employed diisocyanates such as toluene 2,4-diisocyanate (also referred to as 2,4-tolylene diisocyanate, TDI etc.). Later, it was found that higher molecular weight diisocyanates such as hexamethylene diisocyanate (HDI), dimeryl diisocyanate (DDI), or isophorone diisocyanate, gave better properties because of these multi-functional crosslinking agents. The improved properties derived were essential for propellant compositions containing high solids loading. The curing cycles also resulted in better cures as a result of curing catalysts and curing accelerators employed in these types of propellants.
As the burning rate of the propellant has been enhanced to meet the performance requirement of advanced interceptors, particularly for upper-stages where ignition and pressurization (while under high stress conditions) imposes even more requirements for superior strength for the propellant, the need for such superior structural strength for the propellant well beyond the present state-of-the art has been a continued requirement.
The present requirement for solid propellant to withstand the acceleration loads imposed on them during launch of an advanced, high-acceleration interceptor whose mission is to reach a low-altitude intercept point and destroy an incoming intercontinental ballistic missile which has penetrated the defense has created the need for ultrahigh strength mechanical properties well beyond that of the present state-of-the-art propellants.
Studies have been carried out which indicate that current tensile strength of 200 psi and strain of maximum stress of 25% would be far too low by a factor of three for tensile strength, and a factor of two on strain at maximum stress for an unsupported gain.
The state-of-the-art was improved by employing a perforated, support tube which was fitted into the grain's perforation to mechanically reinforce the propellant grain. This feature was particularly advantageous where pressurization and ignition were required for upper stages which were already subjected to extra stresses and forces as a result of high-accelerations and maneuvers of the high-acceleration interceptor.
Achieving tensile strength of 600 psi is unachievable through any modification which has been investigated with state-of-the-art propellants, even with those which have a considerably lower burning rate then that required of interceptor propellants. To further complicate matters relative to propellant mechanical properties, the interceptor propellants contain high percentages of liquid burning rate accelerators, and these liquid burning rate accelerators, which function also as partial plasticizers, tend to degrade the mechanical properties.
An object of this invention is to provide a method of synthesizing solid missile propellants which have structural strengths well beyond those of the state-of-the-art propellants.
A further object of this invention is to provide structurally-strong solid propellants which have structural strengths well beyond those strengths of the state-of-the-art propellants.